The invention pertains to a filter device for filtering fluids.
A filter device used for tangential or crossflow filtration processes, in which a predominantly tangential flow is produced along a filter membrane, is shown in DE 72 25 662 U1.
Another filter device having at least one filter surface for filtration of fluids containing solids is known from DE 39 24 658 A1 ('658). In order to prevent the formation of filter cakes and the clogging of the filter pores, the '658 publication proposes to set the filter surface in motion. Alternatively or additionally, a flow can be produced in the fluid to be filtered, wherein the velocity component of said flow that is directed tangentially relative to the filter surface is no lower than 0.1 m/sec. A turbulent flow is preferably produced in this case in order to keep the filter surface clean, for example, by means of a passing flow consisting of a gas/fluid mixture. However, the desired turbulence for counteracting the formation of filter cakes can also be produced with pumps, nozzles, agitating units, baffles, guide plates, rotating rollers or vibration devices. In addition, it would also be conceivable to electrically charge the filter surface such that it repels solids.
The known filter device comprises several dirty water chambers and a corresponding number of pure water chambers that are separated from one another by a membrane. The main flow direction in the dirty water chambers extends tangentially to the membrane. The fluid to be filtered is also supplied parallel to the membrane. Another important aspect is that the filtration takes place in a largely unpressurized fashion. An enrichment of the solids contained in the fluid occurs in front of the filter surfaces in the dirty water chambers, and the solids are then removed from the filter surfaces by means of lines and pumps. Depending on the turbidity of the fluid, this should take place in the dirty water chambers. A measuring head and a control unit for controlling the corresponding pump are also provided in this case.
Blocking or “fouling” of the filter membranes occurs in membrane filtration methods. The fouling causes the rate of flow through the filter membrane or the filter medium per unit time (dV/dt) to decrease. Various subprocesses contribute individually or collectively to this fouling. First, the pores of the membrane are partially blocked such that a relatively rapid decrease in the rate of flow results; second, a so-called “filter cake” (frequently also referred to as a “secondary membrane”) is formed on the upstream side of the filter medium. This filter cake additionally increases the flow resistance such that the rate of flow through the filter medium is additionally reduced, but it increases the resistance more slowly than does blocking of the pores. A so-called concentration polarization layer may also form on the filter cake.
The various processes that cause fouling can be categorized into reversible and irreversible processes that can either be reversed with certain measures, e.g., backflushing, or not reversed (e.g., adsorption).
In addition, the retained materials not only form a filter cake on the upstream side of the filter (inflow side), but penetrate at least partially into the filter medium and block the channel or pore structures therein. In such instances, cleaning of the filter medium by means of conventional cleaning methods can, if at all, be achieved only with great difficulty.
These mechanisms reduce the maximum operating time (service life) of the filter media and consequently increase the costs and expenditure of labor (maintenance, monitoring, etc.) for the filtration process.
Numerous proposals, methods, and devices for removing a filter cake are known from the prior art. Physical and chemical methods are utilized for removing the retained materials and for cleaning the filter.
The physical cleaning methods include, for example, the use of raking or shaking devices. In methods of this type, the filter cake is removed from the upstream side of the filter medium by a rake or a similar device. Sometimes the filter is set into vibration or a turbulent flow is produced.
For example, baffles and/or frequency generators (such as ultrasound generators) are used to produce turbulent flows. In a few embodiments known from the prior art, a flow that is directed parallel to the filter surface (crossflow) is produced in order to carry off filter cake particles, with the aid of a suitably introduced gas, for example. However, such mechanisms require large filter surfaces for achieving a sufficient flow through the filter medium.
So-called “backwashing” of the filter also falls into this category of cleaning methods. In this method, the filtration process is interrupted, the filter medium is flushed with a cleaning fluid in the direction opposite to the original flow, and the filtration process is subsequently continued (with a certain “lead time”).
In chemical methods, the filter medium is, for example, flushed with one or more cleaning solutions. These cleaning solutions may remain in the filter medium after the cleaning process and, if so required, need to be flushed out (for example, by continuing the filtration process and discarding rather than using fluid filtered immediately after the filtration process is restarted).
Known methods for removing a filter cake (e.g., DE 39 24 658 A1) have certain disadvantages. They require a substantial technical expenditure (raking or agitating units, gas supply, vibration devices, etc.), and the methods become more complicated and susceptible to defects. In addition, the maintenance effort and costs are comparatively high.
Conventional cleaning methods are not always entirely effective, i.e., they do not completely clean the filter membrane or its inflow side.
Another disadvantage can be seen in the fact that certain devices known from the prior art require large systems, filter surfaces, etc., in order to achieve a sufficient filtrate throughput.
Another disadvantage of these cleaning methods is that filtration needs to be interrupted in order to carry out the cleaning process, which reduces filter throughput and increases costs.
Another disadvantage can be seen in the energy requirement of these conventional methods and the associated costs.
According to DE 38 18 437 A1, which also forms part of the pertinent prior art, a filter battery with baffle elements is used in order to produce turbulences. The battery is alternately operated in the filtration mode and in the flushing mode.
DE 43 29 587 C1 discloses a self-cleaning device for filtering a fluid, wherein the filter is integrated into the wall of a pipe and the fluid to be filtered is supplied to the pipe. The fluid to be filtered partially passes through the filter and emerges in the form of filtrate, and the remainder of the fluid to be filtered, which still contains solids, emerges from the outwardly opening end of the pipe.
WO 97/32652 discloses a filter in which the medium to be filtered is also subjected to intense turbulence that is, for example, produced with a rotor, sound waves, electric fields and the like.
DE 35 20 489 C1 discloses a filter with a tangential inlet, wherein the fluid is introduced into the interior of a receptacle with a controlled turbulence and an agitator arranged above the filter membrane produces a continuously changing field of alternating pressure in order to prevent the membrane from becoming covered with particles.
DE 14 36 287 A1 also discloses a tangential filter, in which the filter wall is flushed in order to loosen deposits.